Because of its excellent high temperature strength and creep resistance and resistance to oxidation and corrosion, silicon nitride has been used for many years to make parts such as turbine blades and vanes and internal combustion engine parts and other parts requiring endurance under high temperature conditions.
Generally, it has been the practice in the past to make the parts by using either powdered silicon or powdered silicon nitride as the starting materials. When the starting material is powdered silicon, it has been the practice to combine the silicon powder with a predetermined amount of at least one nitriding agent such as iron oxide and then proceed with what is known as a "reaction bonding" process which generally involves compacting the mix in a die under high pressure at ambient temperature to provide the article and then heating the article in a nitrogen containing gaseous environment at temperatures characteristically ranging from about 1350.degree. C. to 1475.degree. C. for long periods of time which commonly exceeds 20 hours due to the large mass of silicon commonly required to be converted to silicon nitride. Reaction bonding of silicon characteristically produces an article which consists of a porous, principally alpha-phase silicon nitride typically having 80-85% theoretical density. Many different processing routes exist, most requiring a long nitriding cycle of from 25 hr. to several hundred hours. When sintering aids are mixed with the silicon powder prior to reaction bonding, the reaction bonded article can be sintered to a high density after conversion of the silicon to alpha-silicon nitride to obtain mechanical properties similar to a sintered silicon nitride powder such as disclosed in U.S. Pat. No. 4,285,895, the disclosure of which is incorporated herein by reference. One of the advantages of this method is that a low amount of shrinkage occurs providing better dimensional control with less distortion when compared to an article manufactured from sintered silicon nitride powder.
A second method for making parts from silicon nitride is called "hot pressing" which involves starting with a billet of silicon nitride powder commonly including predetermined amounts of one or more "densification aids" such as monovalent metal oxides including MgO, BeO and divalent oxides such as Al.sub.2 O.sub.3 and other oxides or other materials well known to those skilled in the art. In this method, the silicon nitride powder billet is then hot pressed in a graphite die into the article under intermediate pressures at temperatures commonly in the range of about 1650.degree. C. to about 1850.degree. C. and the article is then diamond ground to provide the finished product. This method, while providing superior properties, is characteristically also the most expensive.
A third method is provided by sintering compacted alpha-phase silicon nitride powder which is characteristically performed at a temperature of from about 1650.degree. C. to about 1850.degree. C. of which representative examples are disclosed in U.S. Pat. Nos. 3,992,497; 4,004,937; 4,073,845; and 4,264,547, the disclosures of which are incorporated herein by reference. Although this method produces mechanical properties similar to the "hot pressing" method and is utilized to provide more complicated shapes with less diamond grinding, it characteristically produces about 18% to about 20% shrinkage and significant distortions resulting from minor density variations in the article.
In addition to the use of nitriding agents and densification aids, it has also been common practice to include predetermined amounts of one or more organic binders with the silicon powder or silicon nitride powder to enhance bonding of the powder particles to each other during the process of forming the article.
Examples of processes using silicon nitride as the starting material in conjunction with densification aids and binders are respectively disclosed in U.S. Pat. Nos. 3,950,464; 3,992,497, 4,119,689, 4,164,328; and 4,376,742, the disclosure of which are incorporated herein by reference.
However, none of the previously described processes disclose or suggest the advantages associated with the process of the present invention which greatly reduces nitriding time by first coating or causing silicon to grow on silicon nitride particles then converting the thin silicon coating to silicon nitride during a nitriding process enabling substantial savings in cost and time as well as obtaining better dimensional control during sintering and substantially reducing diamond grinding time to produce the finished article.